Copper-containing catalysts, like nickel catalysts, are used to a considerable extent in industrial processes; they play a significant role, for example, in hydrogenation and dehydrogenation. In this case, the starting material to be converted is passed through the fixed catalyst either in the gaseous state (gas phase catalysis) or in the liquid state (liquid phase catalysis). The catalyst can also be used in a finely divided state as a slurry (suspension catalysis).
Catalysts which, along with copper, also contain chromium have a very broad application range. These catalysts are also known as copper chromite catalysts or Adkins catalysts. However, the use of Adkins catalysts is not without problems since chromium (VI) compounds, which are considered to be carcinogenic and require appropriate safety measures during handling, are used during their preparation. Moreover, relatively large amounts of waste water are produced in the course of the preparation, which waste water is polluted by the presence of copper, chromium (VI), and ammonium compounds. Such waste water is undesirable, since the copper and also the chromium (VI) compounds have a highly toxic effect on microorganisms and can be removed from the waste water only by a complex treatment process.
Besides copper chromite catalysts, nickel catalysts have proven suitable in many respects. However, because of their very high activity, nickel catalysts lead, in particular at relatively high reaction temperatures, to uncontrolled side reactions and secondary reactions, for example cleavages, transalkylations, and/or rearrangements. This favors the formation of undesired byproducts and/or secondary products. Copper catalysts are also known which do not contain chemically bound chromium. However, these catalysts do not have the properties of either the Adkins catalysts or the nickel catalysts.
Thus, DOS 20 56 612 describes a catalyst composed of solid solutions of the series (Cu.sub.x Zn.sub.y)Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O, where x and y can have values from 0.5 to 5.5 and the sum of x and y is 6. The solid solutions are obtained by precipitation at a pH of 4.5 to 5.5 by adding a basic precipitant, for example an aqueous Na.sub.2 CO.sub.3 solution, to an aqueous solution containing copper nitrate, zinc nitrate, and aluminum nitrate. The catalyst in unreduced form containing CuO, ZnO, and Al.sub.2 O.sub.3 is used in the reaction of a gas mixture composed of carbon monoxide, carbon dioxide, and hydrogen to give methanol.
EP 125,689 relates to a catalyst for methanol synthesis containing CuO, ZnO, and Al.sub.2 O.sub.3 having a Cu/Zn atomic ratio between 2.8 and 3.8 (corresponding to 26.9 to 36.5 parts by weight of ZnO per 100 parts by weight of CuO) and an Al.sub.2 O.sub.3 fraction of 8% to 12% by weight. Al.sub.2 O.sub.3 is used as colloidal aluminum hydroxide in the preparation, and Cu and Zn are introduced into the catalyst by precipitation from metal salt solutions. 20% to 40% of the pores have a diameter of 2.0 to 7.5 nm (20 to 75 .ANG.)--corresponding to a pore radius r.sub.p of 1.0 to 3.75 nm (10 to 37.5 .ANG.)--and 60% to 80% of the pores, based in each case on the total number of pores, have a diameter greater than 7.5 nm (75 .ANG.), which corresponds to a pore radius r.sub.p of more than 3.75 nm (37.5 .ANG.). The catalysts described in more detail in the examples have, in the unreduced state, a BET surface area of 100 to 127 m.sup.2 /g, 32% to 42% of the pores have a diameter of 2.0 to 7.5 nm (r.sub.p =1.0 to 3.75 nm), and 57% to 68% of the pores have a diameter larger than 7.5 nm (r.sub.p is greater than 3.75 nm).
As the preceding remarks demonstrate, there is a requirement for a catalyst which can be used, at least within a specific application area, in place of both Adkins catalysts and nickel catalysts. Moreover, problems in the preparation of this catalyst, such as handling carcinogenic chromium (VI) compounds, production of waste water containing pollutants, and disposal of chromium-containing used catalysts, are to be avoided. Furthermore, the novel catalyst is to ensure, in particular at relatively high temperatures, a high conversion with high activity and a high selectivity of the reaction; i.e. the undesired side reactions and secondary reactions typical of nickel catalysts are to be avoided as far as possible.
This applies, inter alia, especially to an important area of application for copper chromite or Adkins catalysts and also nickel catalysts; namely, the catalytic hydrogenation of aldehydes to the corresponding alcohols. In this area, the novel catalyst must be capable of replacing not only the nickel catalysts, but also the copper chromite or Adkins catalysts.